Multi-piece solid golf ball

ABSTRACT

In a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, the core is formed primarily of a base rubber, the envelope layer, intermediate layer and cover are each formed of a resin material, and the envelope layer is formed into two layers—an inner layer and an outer layer. The center and surface hardnesses of the core, the surface hardness of the inner envelope layer-encased sphere, the surface hardness of the outer envelope layer-encased sphere, the surface hardness of the intermediate layer-encased sphere and the surface hardness of the ball together satisfy a specific relationship. This ball exhibits a flight performance capable of satisfying professional golfers and skilled amateurs and possesses an excellent controllability on approach shots, in addition to which it has a good feel at impact and an excellent scuff resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2018-229798 filed in Japan on Dec. 7, 2018, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball having five or more layers, including a core, an inner envelope layer, an outer envelope layer, an intermediate layer and a cover.

BACKGROUND ART

Various golf balls have hitherto been developed for professional golfers and skilled amateurs. Of these, from the standpoint of achieving both a superior distance performance in the high head-speed range and good controllability on shots with an iron and on approach shots, multi-piece solid golf balls having an optimized hardness relationship among the layers encasing the core are in widespread use. Moreover, because not only the flight performance, but also the feel of the ball at impact and the spin rate of the ball after being struck by a club have a large influence on control of the ball, one important topic in golf ball development is optimizing the thicknesses and hardnesses of the golf ball layers in order to achieve the best possible feel and spin rate. Furthermore, there exists a desire for the ball to have durability to repeated impact and for scuffing observed on the ball surface when a golf ball is repeatedly hit with different clubs to be suppressed (increased scuff resistance), maximal protection of the ball from external factors also being an important topic in golf ball development.

Art relating to golf balls having a four-layer construction consisting of a core encased by three layers—an envelope layer, an intermediate layer and a cover (outermost layer)—in which the ball construction is internally varied among the plurality of layers is described in, for example, U.S. Pat. Nos. 7,335,115, 7,918,749, 8,764,584 and 9,174,093.

In addition, art relating to golf balls having a five-layer construction consisting of a core encased by four layers—an inner envelope layer, an outer envelope layer, an intermediate layer and a cover (outermost layer)—in which the ball construction is internally varied among the plurality of layers such that the core surface hardness <inner envelope layer hardness <outer envelope layer hardness <intermediate layer hardness >cover hardness is described in, for example, U.S. Pat. Nos. 7,749,108, 7,445,567, 7,637,826 and 8,371,960. Other golf balls having five or more layers are described in, for example, U.S. Pat. Nos. 8,357,060 and 8,979,677.

However, in these golf balls, there is still room for improvement in optimization of the core hardness profile and the thickness relationship among the layers. That is, as golf ball products for professional golfers and skilled amateurs, there remains room for improvement in achieving an even more improved flight performance and a good feel at impact.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which, along with achieving a flight capable of satisfying professional golfers and skilled amateurs and having an excellent controllability on approach shots, also has a good feel at impact and an excellent scuff resistance.

As a result of extensive investigations, we have discovered that when a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover is constructed such that the core is formed primarily of a base rubber, the envelope layer, intermediate layer and cover are each formed of like or unlike resin materials and the envelope layer is formed as two layers consisting of an inner layer and an outer layer, and the ball is produced such that the surface hardness relationship among these various layers satisfies the following condition:

center hardness of core <surface hardness of core <surface hardness of inner envelope layer-encased sphere >surface hardness of outer envelope layer-encased sphere <surface hardness of intermediate layer-encased sphere >surface hardness of ball, the spin rate of the ball on shots with a driver (W#1) is suppressed and a high initial velocity on shots can be obtained, enabling a good distance to be achieved. In addition, the spin rate on approach shots in the short game is optimized, increasing the ball controllability. Moreover, on shots with a driver (W#1), the ball has a good feel that is not too hard.

That is, the golf ball of the invention, by being able to hold down the spin rate of the ball on shots with a driver (W#1) and by having a high initial velocity on shots, is able to achieve a good distance. Generally, when a golf ball is designed so as to favor distance, the ball tends to have too hard at feel at impact. However, in the present invention, by giving the golf ball a multilayer structure and specifying the surface hardnesses of the respective layer-encased spheres therein, the ball can be endowed with both a good distance and a good feel, making the ball ideally suited for professional golfers and skilled to amateurs. Furthermore, in this invention, in order for the ball to fully exhibit both a high controllability in the short game and an excellent scuff resistance, it is preferable to use as the outermost layer material a urethane resin material having flexibility and a high durability.

Accordingly, in a first aspect, the invention provides a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber; the envelope layer, intermediate layer and cover are each formed of a resin material; the envelope layer is formed into two layers-an inner layer and an outer layer; and the core has a center hardness and a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy the following relationship:

core center hardness<core surface hardness<surface hardness of inner envelope layer-encased sphere>surface hardness of outer envelope layer-encased sphere<surface hardness of intermediate layer-encased sphere>ball surface hardness.

In a preferred embodiment of the multi-piece solid golf ball according to the first aspect of the invention, the core has a hardness profile in which, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the center and the surface of the core, C_(M+2.5). C_(M+5.0) and C_(M+0.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the following surface areas A to F:

½×2.5×(C _(M−5.0) −C _(M−7.5))  surface area A:

½×2.5×(C _(M−2.5) −C _(M−5.0))  surface area B:

½×2.5×(C _(M) −C _(M−2.5))  surface area C:

½×2.5×(C _(M+2.5) −C _(M))  surface area D:

½×2.5×(C _(M+5) −C _(M+2.5))  surface area E:

½×2.5×(C _(M+7.5) −C _(M+5))  surface area F:

satisfy the condition

(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)≥3.

In this preferred embodiment, surface areas A to E in the core hardness profile may satisfy the condition

(surface area D+surface area E)−(surface area A+surface area B+surface area C)≥0.

In the same preferred embodiment, surface areas A to F in the core hardness profile may satisfy the condition

0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.

In the same preferred embodiment, surface areas B to E in the core hardness profile may satisfy the condition

surface area B<surface area C≤surface area D<surface area E.

In another preferred embodiment of the multi-piece solid golf ball of the invention, the envelope layer has a thickness which satisfies the condition

thickness of inner envelope layer≥thickness of outer envelope layer.

In yet another preferred embodiment, the thicknesses of the outer envelope layer and the intermediate layer satisfy the condition

thickness of intermediate layer≥thickness of outer envelope layer.

In still another preferred embodiment, the thicknesses of the envelope layer, the intermediate layer and the cover satisfy the condition

(combined thickness of inner envelope layer and outer envelope layer)−(combined thickness of intermediate layer and cover)≥0.1 mm.

In a further preferred embodiment, a coating layer is formed on a surface of the cover, which coating layer has a Shore C hardness of from 40 to 80. In this preferred embodiment, letting He be the Shore C hardness of the coating layer and letting C_(M) be the Shore C hardness at a midpoint M between the center and surface of the core, the difference C_(M)−Hc may be at least −10 and up to 10.

In a second aspect, the invention provides a multi-piece solid golf ball having a core, an inner envelope layer, an outer envelope layer, an intermediate layer and a cover, wherein the core has a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy the following relationship:

core surface hardness <surface hardnesses of the inner envelope layer-encased sphere, outer envelope layer-encased sphere and intermediate layer-encased sphere >ball surface hardness,

with the proviso that the surface hardness of the outer envelope layer-encased sphere is not more than 60 on the Shore D hardness scale and the surface hardness of the intermediate layer-encased sphere is at least 66 on the Shore D hardness scale; and the core has a hardness profile in which, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the center and the surface of the core, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the following surface areas A to F:

½×2.5×(C _(M−5.0) −C _(M−7.5))  surface area A:

½×2.5×(C _(M−2.5) −C _(M−5.0))  surface area B:

½×2.5×(C _(M) −C _(M−2.5))  surface area C:

½×2.5×(C _(M+2.5) −C _(M))  surface area D:

½×2.5×(C _(M+5) −C _(M+2.5))  surface area E:

½×2.5×(C _(M+7.5) −C _(M+5))  surface area F:

satisfy the condition

(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)≥6.

In a preferred embodiment of the multi-piece solid golf ball according to the second aspect of the invention, surface areas A to F in the core hardness profile satisfy the condition

0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.

In another preferred embodiment of the golf ball according to the second aspect of to the invention, a coating layer is formed on a surface of the cover and, letting He be the Shore C hardness of the coating layer and letting Chi be the Shore C hardness at the midpoint M between the center and surface of the core, the difference C_(M)−Hc is at least −10 and up to 10.

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention has a lowered spin rate on full shots with a driver, enabling the distance of the ball to be further increased, and also has a good controllability on approach shots. In addition, the feel at impact is good and the scuff resistance is excellent. Such qualities make this ball highly useful as a golf ball for professional golfers and skilled amateurs.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of the multi-piece solid golf ball (5-layer structure) according to the invention.

FIG. 2 is a graph that uses core hardness profile data from Example 1 to explain surface areas A to F in the core hardness profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagrams.

The multi-piece solid golf ball of the invention is, as shown in FIG. 1, a multilayer golf ball G having four or more layers that include a core 1, an inner envelope layer 2 a and outer envelope layer 2 b encasing the core 1, an intermediate layer 3 encasing the envelope layers, and a cover 4 encasing the intermediate layer 3. Numerous dimples D are typically formed on the surface of the cover 4. Although not shown in the diagram, a coating layer is generally coated onto the surface of the cover 4. Excluding the coating layer, the cover 4 is positioned as the outermost layer in the layered structure of the golf ball. The core 1, intermediate layer 3 and cover 4 are each not limited to a single layer and may be formed of a plurality of two or more layers.

The core has a diameter which, although not particularly limited, is preferably at least 30.0 mm, more preferably at least 31.0 mm, and even more preferably at least 31.5 mm. The core diameter is preferably not more than 35.0 mm, more preferably not more than 34.0 mm, and even more preferably not more than 33.5 mm. When the core diameter is too small, the spin rate on shots with a driver (W#1) may rise, as a result of which the desired distance may not be achieved. On the other hand, when the core diameter is too large, the durability to repeated impact may worsen or the ball may have a poor feel at impact.

The core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably at least 3.0 mm, more preferably at least 3.5 mm, and even more preferably at least 4.0 mm. The core deflection is preferably not more than 7.0 mm, more preferably not more than 6.0 mm, and even more preferably not more than 5.0 mm. When the core deflection is too small, i.e., when the core is too hard, the spin rate of the ball may rise excessively, possibly resulting in a poor distance, or the feel at impact may be too hard. On the other hand, when the core deflection is too large, i.e., when the core is too soft, the ball rebound may be too low, resulting in a poor distance, the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

The core material is made primarily of a rubber material. Specifically, a rubber composition can be prepared using a base rubber as the primary component and blending with this other ingredients such as co-crosslinking agents, organic peroxides, inert fillers and organosulfur compounds. It is preferable to use polybutadiene as the base rubber.

Commercial products may be used as the polybutadiene. Illustrative examples include BR01, BR51 and BR730 (all products of JSR Corporation). The proportion of polybutadiene within the base rubber is preferably at least 60 wt %, and more preferably at least 80 wt %. Rubber ingredients other than the above polybutadienes may be included in the base rubber, provided that doing so does not detract from the advantageous effects of the invention. Examples of rubber ingredients other than the above polybutadienes include other polybutadienes and also other diene rubbers, such as styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

Examples of co-crosslinking agents include unsaturated carboxylic acids and metal salts of unsaturated carboxylic acids. Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. Metal salts of unsaturated carboxylic acids are exemplified by, without particular limitation, the above unsaturated carboxylic acids that have been neutralized with desired metal ions. Specific examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, which is typically at least 5 parts by weight, preferably at least 9 parts by weight, and more preferably at least 13 parts by weight.

The amount included is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel at impact, whereas too little may lower the rebound.

Commercial products may be used as the organic peroxide. Examples of such products that may be suitably used include Percumyl D, Perhexa C-40 and Perhexa 3M (all from NOF Corporation), and Luperco 231XL (from AtoChem Co.). One of these may be used alone, or two or more may be used together. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, even more preferably at least 0.5 part by weight, and most preferably at least 0.6 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2.5 parts by weight. When too much or too little is included, it may not be possible to obtain a ball having a good feel, durability and rebound.

Another compounding ingredient typically included with the base rubber is an inert filler, preferred examples of which include zinc oxide, barium sulfate and calcium carbonate. One of these may be used alone, or two or more may be used together. The amount of inert filler included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 5 parts by weight. The upper limit is preferably not more than 50 parts by weight, more preferably not more than 40 parts by weight, and even more preferably not more than 36 parts by weight. Too much or too little inert filler may make it impossible to obtain a proper weight and a suitable rebound.

In addition, an antioxidant may be optionally included. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6 and Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). One of these may be used alone, or two or more may be used together.

The amount of antioxidant included per 100 parts by weight of the base rubber is set to 0 part by weight or more, preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight. The upper limit is set to preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, even more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a suitable ball rebound and durability.

An organosulfur compound may be included in the core in order to impart a good resilience. The organosulfur compound is not particularly limited, provided it can enhance the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts of these. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfides, dibenzvlpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. The zinc salt of pentachlorothiophenol is especially preferred.

The amount of organosulfur compound included per 100 parts by weight of the base rubber is 0 part by weight or more, and it is recommended that the amount be preferably at least 0.05 part by weight, and even more preferably at least 0.1 part by weight, and that the upper limit be preferably not more than 5 parts by weight, more preferably not more than 3 parts by weight, and even more preferably not more than 2.5 parts by weight. Including too much organosulfur compound may make a greater rebound-improving effect (particularly on shots with a W#1) unlikely to be obtained, may make the core too soft or may worsen the feel of the ball at impact. On the other hand, including too little may make a rebound-improving effect unlikely.

Decomposition of the organic peroxide within the core formulation can be promoted by the direct addition of water (or a water-containing material) to the core material. The decomposition efficiency of the organic peroxide within the core-forming rubber composition is known to change with temperature, starting at a given temperature, the decomposition efficiency rises with increasing temperature. If the temperature is too high, the amount of decomposed radicals rises excessively, leading to recombination between radicals and, ultimately, deactivation. As a result, fewer radicals act effectively in crosslinking. Here, when a heat of decomposition is generated by decomposition of the organic peroxide at the time of core vulcanization, the vicinity of the core surface remains at substantially the same temperature as the vulcanization mold, but the temperature near the core center, due to the build-up of heat of decomposition by the organic peroxide which has decomposed from the outside, becomes considerably higher than the mold temperature. In cases where water (or a water-containing material) is added directly to the core, because the water acts to promote decomposition of the organic peroxide, radical reactions like those described above can be made to differ at the core center and core surface. That is, decomposition of the organic peroxide is further promoted near the center of the core, bringing about greater radical deactivation, which leads to a further decrease in the amount of active radicals. As a result, it is possible to obtain a core in which the crosslink densities at the core center and the core surface differ markedly. It is also possible to obtain a core having different dynamic viscoelastic properties at the core center.

The water included in the core material is not particularly limited, and may be distilled water or tap water. The use of distilled water that is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

The core can be produced by vulcanizing and curing the rubber composition containing the above ingredients. For example, the core can be produced by using a Banbury mixer, roll mill or other mixing apparatus to intensively mix the rubber composition, subsequently compression molding or injection molding the mixture in a core mold, and curing the resulting molded body by suitably heating it under conditions sufficient to allow the organic peroxide or co-crosslinking agent to act, such as at a temperature of between 100 and 200° C., preferably between 140 and 180° C., for 10 to 40 minutes.

The core may consist of a single layer alone, or may be formed as a two-layer core consisting of an inner core layer and an outer core layer. When the core is formed as a two-layer core consisting of an inner core layer and an outer core layer, the inner core layer and outer core layer materials may each be composed primarily of the above-described to rubber material. The rubber material making up the outer core layer encasing the inner core layer may be the same as or different from the inner core layer material. The details here are the same as those given above for the ingredients of the core-forming rubber material.

Next, the core hardness profile is described. In the explanation below, the core hardness refers to the Shore C hardness. This Shore C hardness is a hardness value measured with a Shore C durometer in general accordance with ASTM D2240.

The core has a center hardness (Cc) which is preferably at least 50, more preferably at least 52, and even more preferably at least 54. The upper limit is preferably not more than 59, more preferably not more than 57, and even more preferably not more than 55. When this value is too large, the feel at impact may harden, or the spin rate on full shots may rise, as a result of which the intended distance may not be achieved. On the other hand, when this value is too small, the rebound may become lower and so a good distance may not be obtained, or the durability to cracking under repeated impact may worsen.

The core has a surface hardness (Cs) which is preferably at least 73, more preferably at least 75, and even more preferably at least 77. The upper limit is preferably not more than 85, more preferably not more than 83, and even more preferably not more than 81. A core surface hardness outside of this range may lead to undesirable results similar to those described above for the core center hardness (Cc).

The difference between the core surface hardness (Cs) and the core center hardness (Cc) is preferably at least 20, more preferably at least 22, and even more preferably at least 24. The upper limit is preferably not more than 35, more preferably not more than 32, and even more preferably not more than 28. When this value is too small, the ball spin rate-lowering effect on shots with a driver may be inadequate, resulting in a poor distance. When this value is too large, the initial velocity of the ball when struck may decrease, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

In the above core hardness profile in this invention, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the center and the surface of the core, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the following surface areas A to F:

½×2.5×(C _(M−5.0) −C _(M−7.5))  surface area A:

½×2.5×(C _(M−2.5) −C _(M−5.0))  surface area B:

½×2.5×(C_(M)−C_(M−2.5))  surface area C:

½×2.5×(C _(M+2.5) −C _(M))  surface area D:

½×2.5×(C _(M+5) −C _(M+2.5))  surface area E:

½×2.5×(C _(M+7.5) −C _(M+5))  surface area F:

are preferably such that the value of (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C) satisfies the specific range described below. FIG. 2 shows a graph that uses core hardness profile data from Example 1 to explain surface areas A to F. As is apparent from the graph, each of surface areas A to F is the surface area of a triangle whose base is the difference between specific distances in the core cross-section and whose height is the difference in hardness between positions at these specific distances.

The value of (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C) above is preferably at least 3, more preferably at least 4, and even more preferably at least 6. The upper limit is preferably not more than 20, more preferably not more than 15, and even more preferably not more than 10. When this value is too small, the spin rate lowering effect on shots with a driver (W#1) may be inadequate, as a result of which a good distance may not be achieved. When this value is too large, the initial velocity of the ball when struck may become lower, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

In the above core hardness profile, it is preferable for the following condition to be satisfied: 0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.60. The lower limit value here is preferably at least 0.20, and more preferably at least 0.25. The upper limit value in this formula is preferably not more than 0.50, and more preferably not more than 0.40. When this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and so a good distance may not be achieved. On the other hand, when this value is too large, the initial velocity of the ball when struck may be low, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

In addition, in the above core hardness profile, it is preferable for the following condition to be satisfied: −2≤(surface area D+surface area E)−(surface area A+surface area B+surface area C)≤8. The lower limit value here is more preferably at least 0, and even more preferably at least 1. The upper limit value is more preferably not more than 6, and even more preferably not more than 4. When this value is too small, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate, and so a good distance may not be achieved. On the other hand, when this value is too large, the initial velocity of the ball when struck may become lower, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

Surface areas B to E in the core hardness profile preferably satisfy the relationship: surface area B<surface area C≤surface area D<surface area E. When this relationship is not satisfied, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and so a good distance may not be achieved.

Next, the envelope layer is described.

In this invention, the envelope layer is formed of two layers: an inner layer and an outer layer. These are referred to as, respectively, the inner envelope layer and the outer envelope layer.

The inner envelope layer has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 46, more preferably at least 48, and even more preferably at least 50. The upper limit is preferably not more than 60, more preferably not more than 58, and even more preferably not more than 56. The sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness on the Shore D scale which is preferably at least 52, more preferably at least 54, and even more preferably at least 56. The upper limit is preferably not more than 66, more preferably not more than 64, and even more preferably not more than 62. When the material hardness and surface hardness of the inner envelope layer are lower than the above ranges, the spin rate of the ball on full shots may rise, as a result of which the intended distance may not be achieved. On the other hand, when the material hardness and surface hardness are too high, the feel at impact may become too hard or the durability to cracking on repeated impact may worsen.

The inner envelope layer has a thickness which is preferably at least 0.8 mm, more preferably at least 1.0 mm, and even more preferably at least 1.2 mm. The upper limit in the thickness of the inner envelope layer is preferably not more than 1.8 mm, more preferably not more than 1.6 mm, and even more preferably not more than 1.4 mm. When the inner envelope layer thickness falls outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved.

The outer envelope layer has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 43, more preferably at least 45, and even more preferably at least 47. The upper limit is preferably not more than 54, more preferably not more than 52, and even more preferably not more than 50. The sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) has a surface hardness on the Shore D scale which is preferably at least 49, more preferably at least 51, and even more preferably at least 53. The upper limit is preferably not more than 60, more preferably not more than 58, and even more preferably not more than 56. When the material hardness and surface hardness of the outer envelope layer are lower than the above ranges, the spin rate of the ball on full shots may rise, as a result of which the intended distance may not be achieved. On the other hand, when the material hardness and surface hardness are too high, the feel at impact may become too hard or the durability to cracking on repeated impact may worsen.

The outer envelope layer has a thickness which is preferably at least 0.8 mm, more preferably at least 1.0 mm, and even more preferably at least 1.1 mm. The upper limit in the thickness of the outer envelope layer is preferably not more than 1.7 mm, more preferably not more than 1.5 mm, and even more preferably not more than 1.3 mm. When the outer envelope layer thickness falls outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved.

The materials making up the inner envelope layer and the outer envelope layer are not particularly limited; known resins may be used for this purpose. Examples of preferred materials include resin compositions containing as the essential ingredients:

100 parts by weight of a resin component composed of, in admixture,

(A) a base resin of (a-1) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer mixed with (a-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and

(B) a non-ionomeric thermoplastic elastomer

in a weight ratio between 100:0 and 50:50;

(C) from 5 to 120 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1,500; and

(D) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in components A and C.

Components A to D in the intermediate layer-forming resin material described in, for example, JP-A 2010-253268 may be advantageously used as above components A to D.

The resin materials that form the inner envelope layer and the outer envelope layer may be mutually like or unlike.

A non-ionomeric thermoplastic elastomer may be included in the respective materials for the inner envelope layer and the outer envelope layer. The non-ionomeric thermoplastic elastomer is preferably included in an amount of from 0 to 50 parts by weight per 100 parts by weight of the total amount of the base resin.

Exemplary non-ionomeric thermoplastic elastomers include polyolefin elastomers (including polyolefins and metallocene polyolefins), polystyrene elastomers, diene polymers, polyacrylate polymers, polyamide elastomers, polyurethane elastomers, polyester elastomers and polyacetals.

Optional additives may be suitably included in the above resin materials. For example, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When these additives are included, the amount added per 100 parts by weight of the overall base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

Next, the intermediate layer is described.

The intermediate layer has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 60, more preferably at least 63, and even more preferably at least 65. The upper limit is preferably not more than 72, more preferably not more than 70, and even more preferably not more than 68. The surface hardness of the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere), expressed on the Shore D scale, is preferably at least 66, more preferably at least 69, and even more preferably at least 71. The upper limit is preferably not more than 78, more preferably not more than 76, and even more preferably not more than 74. When the material hardness and surface hardness of the intermediate layer are lower than the above respective ranges, the ball rebound on full shots may be inadequate or the spin rate on full shots may rise excessively, resulting in a poor distance. On the other hand, when the material hardness and surface hardness are too high, the durability to cracking on repeated impact may worsen or the feel at impact may end up becoming too hard.

The intermediate layer has a thickness of preferably at least 0.8 mm, more preferably at least 1.0 mm, and even more preferably at least 1.1 mm. The upper limit in the intermediate layer thickness is preferably not more than 1.7 mm, more preferably not more than 1.5 mm, and even more preferably not more than 1.3 mm. It is preferable for the intermediate layer to have a greater thickness than the subsequently described cover (outermost layer). When the intermediate layer thickness falls outside of the above range or the intermediate layer is formed so as to be thinner than the cover, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved.

Various types of thermoplastic resins, particularly ionomeric resins, that are used as golf ball materials may be suitably used as the intermediate layer material. Commercial products may be used as the ionomeric resin. Alternatively, the intermediate layer-forming resin material that is used may be one obtained by blending, of commercially available ionomeric resins, a high-acid ionomeric resin having an acid content of at least 16 wt % into a conventional ionomeric resin. The high rebound and spin rate-lowering effect obtained with such a blend make it possible to achieve a good distance on shots with a driver (W#1).

The amount of unsaturated carboxylic acid included in the high-acid ionomeric resin (acid content) is typically at least 16 wt %, preferably at least 17 wt %, and more preferably at least 18 wt %. The upper limit is preferably not more than 22 wt %, more preferably not more than 21 wt %, and even more preferably not more than 20 wt %. When this value is too small, the spin rate on full shots may rise, as a result of which the intended distance may not be achieved. On the other hand, when this value is too large, the feel at impact may be too hard or the durability to cracking on repeated impact may worsen.

The amount of high-acid ionomeric resin per 100 wt % of the resin material is preferably at least 10 wt %, more preferably at least 30 wt %, and even more preferably at least 60 wt %. When the amount of such high-acid ionomeric resin included is too low, the spin rate on shots with a driver (W#1) may be high, as a result of which a good distance may not be achieved.

Depending on the intended use, optional additives may be suitably included in the intermediate layer material. For example, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When these additives are included, the amount added per 100 parts by weight of the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

It is desirable to abrade the surface of the intermediate layer in order to increase adhesion of the intermediate layer material with the polyurethane that is preferably used in the subsequently described cover material. In addition, it is desirable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion treatment or to add an adhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which is typically less than 1.1, preferably between 0.90 and 1.05, and more preferably between 0.93 and 0.99. Outside of this range, the rebound of the overall ball may decrease and a good distance may not be obtained, or the durability of the ball to cracking on repeated impact may worsen.

Next, the cover is described.

The cover has a material hardness on the Shore D scale which, although not particularly limited, is preferably at least 35, and more preferably at least 40. The upper limit is preferably not more than 55, more preferably not more than 53, and even more preferably not more than 50. The surface hardness of the sphere obtained by encasing the intermediate layer-encased sphere with the cover (i.e., the ball), expressed on the Shore D scale, is preferably at least 50, more preferably at least 55, and even more preferably at least 58. The upper limit is preferably not more than 66, more preferably not more than 64, and even more preferably not more than 63. When the material hardness of the cover and the surface hardness of the ball are too much lower than the above respective ranges, the spin rate of the ball on shots with a driver (W#1) may rise and a good distance may not be achieved. On the other hand, when the material hardness of the cover and the surface hardness of the ball are too high, the ball controllability in the short game may worsen or the scuff resistance may worsen.

The cover has a thickness of preferably at least 0.3 mm, more preferably at least 0.45 mm, and even more preferably at least 0.6 mm. The upper limit in the cover thickness preferably not more than 1.2 mm, more preferably not more than 1.0 mm, and even more preferably not more than 0.8 mm. Also, it is preferable for the cover to have a lower thickness than the intermediate layer. When the cover has a thickness outside of the above range or is thicker than the intermediate layer, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved.

Various types of thermoplastic resins employed as cover stock in golf balls may be used as the cover material. For reasons having to do with ball controllability and scuff resistance, preferred use can be made of a urethane resin. In particular, from the standpoint of the mass productivity of the manufactured balls, it is preferable to use a material that is composed primarily of a thermoplastic polyurethane, and especially preferable to form the cover of a resin composition in which the main components are (I) a thermoplastic urethane and (II) a polyisocyanate compound.

It is recommended that the total weight of components (I) and (II) combined be at least 60%, and preferably at least 70%, of the overall amount of the cover-forming resin composition. Components (1) and (II) are described below.

The thermoplastic polyurethane (I) has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate compound. Here, the long-chain polyol serving as a starting material may be any that has hitherto been used in the art relating to thermoplastic polyurethanes, and is not particularly limited. Illustrative examples include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly, or two or more may be used in combination. Of these, in terms of being able to synthesize a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties, a polyether polyol is preferred.

Any chain extender that has hitherto been employed in the art relating to thermoplastic polyurethanes may be suitably used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, the chain extender is preferably an aliphatic diol having 2 to 12 carbon atoms, and more preferably 1,4-butylene glycol.

Any polyisocyanate compound hitherto employed in the art relating to thermoplastic polyurethanes may be suitably used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reactions during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

Commercially available products may be used as the thermoplastic polyurethane serving as component (I). Illustrative examples include Pandex T-8295, Pandex T-8290 and Pandex T-8260 (all from DIC Covestro Polymer, Ltd.).

A thermoplastic elastomer other than the above thermoplastic polyurethanes may also be optionally included as a separate component, i.e., component (III), together with above components (I) and (II). By including this component (III) in the above resin blend, the flowability of the resin blend can be further improved and properties required of the golf ball cover material, such as resilience and scuff resistance, can be increased.

The compositional ratio of above components (I). (II) and (III) is not particularly limited. However, to fully elicit the advantageous effects of the invention, the compositional ratio (I):(II):(III) is preferably in the weight ratio range of from 100:2:50 to 100:50:0, and more preferably from 100:2:50 to 100:30:8.

In addition, various additives other than the ingredients making up the above thermoplastic polyurethane may be optionally included in this resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and internal mold lubricants may be suitably included.

The manufacture of multi-piece solid golf balls in which the above-described core, inner envelope layer, outer envelope layer, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection molding process. For example, a multi-piece golf ball can be produced by successively injection-molding the respective materials for the inner envelope layer, outer envelope layer and intermediate layer over the core in injection molds for each layer so as to obtain the respective layer-encased spheres and then, last of all, injection-molding the material for the cover serving as the outermost layer over the intermediate layer-encased sphere. Alternatively, the encasing layers may each be formed by enclosing the sphere to be encased within two half-cups that have been pre-molded into hemispherical shapes and then molding under applied heat and pressure.

Hardness Relationships Among Layers

In the first aspect of this invention, it is critical that the core have a center hardness and a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) have a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) have a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) have a surface hardness and the ball have a surface hardness which together satisfy the following relationship:

core center hardness<core surface hardness<surface hardness of inner envelope layer-encased sphere>surface hardness of outer envelope layer-encased sphere<surface hardness of intermediate layer-encased sphere>ball surface hardness.

When this hardness relationship is not satisfied, a sufficient flight performance on shots with a driver (W#1) is not obtained or the feel at impact is too hard. In addition, the spin rate on approach shots may be lower and the controllability may worsen.

From the standpoint of suppressing the spin rate on shots with a driver (W#1) and ensuring a good distance, the above hardness relationship preferably satisfies the following condition:

(core surface hardness)≤(surface hardness of outer envelope layer-encased sphere).

Thickness Relationships Among Layers

In this invention, to achieve both a good flight and a good controllability in the short game, it is preferable for the thicknesses of the inner and outer envelope layers to satisfy the relationship (thickness of inner envelope layer)≥(thickness of outer envelope layer), more preferable for the inner envelope layer to be thicker than the outer envelope layer, and still more preferable for the inner envelope layer to be thicker than the intermediate layer.

The thicknesses of the outer envelope layer and the intermediate layer preferably satisfy the relationship (thickness of intermediate layer)≥(thickness of outer envelope layer). It is especially preferable for the intermediate layer to be thicker than the outer envelope layer.

As mentioned above, it is preferable for the intermediate layer to be formed thicker than the cover. In this case, the difference in thickness between the intermediate layer and the cover is preferably at least 0.2 mm and not more than 1.2 mm. Outside of this range, it may not be possible to achieve both a good distance and a good controllability in the short game.

In addition, regarding the thicknesses of the envelope layers, the intermediate layer and the cover, it is preferable for the value expressed as (combined thickness of inner envelope layer and outer envelope layer)−(combined thickness of intermediate layer and cover) to be at least 0.1 mm, preferably at least 0.3 mm, and more preferably at least 0.4 mm. The upper limit is preferably not more than 1.0 mm, more preferably not more than 0.8 mm, and even more preferably not more than 0.6 mm. Outside of this range, a good distance and a good controllability in the short game may not both be achieved.

Numerous dimples may be formed on the outside surface of the cover serving as the outermost layer. The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 250, more preferably at least 300, and even more preferably at least 320. The upper limit is preferably not more than 380, more preferably not more than 350, and even more preferably not more than 340. When the number of dimples is higher than this range, the ball trajectory may become lower and the distance traveled by the ball may decrease. On the other hand, when the number of dimples is lower that this range, the ball trajectory may become higher and a good distance may not be achieved.

The dimple shapes used may be of one type or may be a combination of two or more types suitably selected from among, for example, circular shapes, various polygonal shapes, dewdrop shapes and oval shapes. When circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.08 mm and up to 0.30 mm.

In order for the aerodynamic properties to be fully manifested, it is desirable for the dimple coverage ratio on the spherical surface of the golf ball, i.e., the dimple surface coverage SR, which is the sum of the individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, as a percentage of the spherical surface area of the ball were the ball to have no dimples thereon, to be set to at least 70% and not more than 90%. Also, to optimize the ball trajectory, it is desirable for the value V₀, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and not more than 0.80. Moreover, it is preferable for the ratio VR of the sum of the volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of the ball sphere were the ball surface to have no dimples thereon, to be set to at least 0.6% and not more than 1.0%. Outside of the above ranges in these respective values, the resulting trajectory may not enable a good distance to be achieved and so the ball may fail to travel a fully satisfactory distance.

A coating layer may be formed on the surface of the cover. This coating layer can be formed by applying various types of coating materials. Because the coating layer must be capable of enduring the harsh conditions of golf ball use, it is desirable to use a coating composition in which the chief component is a urethane coating material composed of a polyol and a polyisocyanate.

The polyol component is exemplified by acrylic polyols and polyester polyols. These polyols include modified polyols. To further increase workability, other polyols may also be added.

It is suitable to use two types of polyester polyols together as the polyol component. In this case, letting the two types of polyester polyol be component (a) and component (b), a polyester polyol in which a cyclic structure has been introduced onto the resin skeleton may be used as the polyester polyol of component (a). Examples include polyester polyols obtained by the polycondensation of a polyol having an alicyclic structure, such as cyclohexane dimethanol, with a polybasic acid; and polyester polyols obtained by the polycondensation of a polyol having an alicyclic structure with a diol or triol and a polybasic acid. A polyester polyol having a branched structure may be used as the polyester polyol of component (b). Examples include polyester polyols having a branched structure, such as NIPPOLAN 800, from Tosoh Corporation.

The polyisocyanate is exemplified without particular limitation by commonly used aromatic, aliphatic, alicyclic and other polyisocyanates. Specific examples include tolylene diisocyanate, diphenylmethane diisocyanate, xylylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, 1,4-cyclohexylene diisocyanate, naphthalene diisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate and 1-isocyanato-3,3,5-trimethyl-4-isocyanatomethylcyclohexane. These may be used singly or in admixture.

Depending on the coating conditions, various types of organic solvents may be mixed into the coating composition. Examples of such organic solvents include aromatic solvents such as toluene, xylene and ethylbenzene; ester solvents such as ethyl acetate, butyl acetate, propylene glycol methyl ether acetate and propylene glycol methyl ether propionate; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ether solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether and dipropylene glycol dimethyl ether; alicyclic hydrocarbon solvents such as cyclohexane, methyl cyclohexane and ethyl cyclohexane: and petroleum hydrocarbon solvents such as mineral spirits.

The thickness of the coating layer made of the coating composition, although not particularly limited, is typically from 5 to 40 μm, and preferably from 10 to 20 μm. As used herein, “coating layer thickness” refers to the coating thickness obtained by averaging the measurements taken at a total of three places: the center of a dimple and two places located at positions between the dimple center and the dimple edge.

In this invention, the coating layer composed of the above coating composition has an elastic work recovery that is preferably at least 60%, and more preferably at least 80%. At a coating layer elastic work recovery in this range, the coating layer has a high elasticity and so the self-repairing ability is high, resulting in an outstanding abrasion resistance. Moreover, the performance attributes of golf balls coated with this coating composition can be improved. The method of measuring the elastic work recovery is described below.

The elastic work recovery is one parameter of the nanoindentation method for evaluating the physical properties of coating layers, this being a nanohardness test method that controls the indentation load on a micro-newton (μN) order and tracks the indenter depth during indentation to a nanometer (nm) precision. In prior methods, only the size of the deformation (plastic deformation) mark corresponding to the maximum load could be measured. However, in the nanoindentation method, the relationship between the indentation load and the indentation depth can be obtained by continuous automated measurement. Hence, unlike in the past, there are no individual differences between observers when visually measuring a deformation mark under an optical microscope, thus enabling the physical properties of the coating layer to be measured reliably and to a high precision. Given that the coating layer on the ball surface is strongly affected by the impact of drivers and various other clubs and has a not inconsiderable influence on various golf ball properties, measuring the coating layer by the nanohardness test method and carrying out such measurement to a higher precision than in the past is a very effective method of evaluation.

The hardness of the coating layer, expressed on the Shore M hardness scale, is preferably at least 40, and more preferably at least 60. The upper limit is preferably not more than 95, and more preferably not more than 85. This Shore M hardness is obtained in general accordance with ASTM D2240. The hardness of the coating layer, expressed on the Shore C hardness scale, is preferably at least 40 and has an upper limit of preferably not more than 80. This Shore C hardness is obtained in general accordance with ASTM D2240. At coating layer hardnesses that are higher than these ranges, the coating may become brittle when the ball is repeatedly struck, which may make it incapable of protecting the cover layer. On the other hand, coating layer hardnesses that are lower than the above range are undesirable because the ball surface more readily incurs damage upon striking a hard object.

In order for the ball to be endowed with both a good flight and a good spin performance on approach shots, letting He be the Shore C hardness of the coating layer, the difference between the Shore C hardness C_(M) at the midpoint M between the core center and surface and Hc (C_(M)−Hc) is preferably −10 or more, and more preferably −5 or more. The upper limit is preferably not more than 10, and more preferably not more than 5.

When the above coating composition is used, the formation of a coating layer on the surface of golf balls manufactured by a commonly known method can be carried out via the steps of preparing the coating composition at the time of application, applying the composition to the golf ball surface by a conventional coating operation, and drying the applied composition. The coating method is not particularly limited. For example, spray painting, electrostatic painting or dipping may be suitably used.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 4, Comparative Examples 1 to 8 Formation of Core

Solid cores were produced by preparing rubber compositions for the respective Examples and Comparative Examples shown in Table 1, and then molding and vulcanizing the compositions under vulcanization conditions of 155° C. and 15 minutes.

TABLE 1 Core formulation Example Comparative Example (pbw) 1 2 3 4 1 2 3 4 5 6 7 8 Polybutadiene A 80 80 Polybutadiene B 20 20 20 20 20 20 20 20 20 20 20 20 Polybutadiene C 80 80 80 80 80 80 80 80 80 80 Zinc acrylate 37.0 34.9 37.0 34.9 37.0 37.0 36.0 37.0 37.0 37.0 22.4 20.3 Organic peroxide (1) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.6 0.6 Organic peroxide (2) 0.6 0.6 Water 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Barium sulfate 40.9 41.7 Zinc oxide 34.8 35.5 34.8 35.5 28.6 19.9 35.5 34.3 34.3 34.3 4.0 4.0 Zinc salt of pentachlorothiophenol 1.0 1.0 1.0 1.0 1.0 1.0 0.2 1.0 1.0 1.0

Details on the ingredients mentioned in Table 1 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR     Corporation -   Polybutadiene B: Available under the trade name “BR 51” from JSR     Corporation -   Polybutadiene C: Available under the trade name “BR 730” from JSR     Corporation -   Zinc acrylate: “ZN-DA85S” from Nippon Shokubai Co., Ltd. -   Organic Peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic Peroxide (2): A mixture of 1,1-di(t-butylperoxy)cyclohexane     and silica, available under the trade name “Perhexa C-40” from NOF     Corporation -   Water: Pure water (from Seiki Chemical Industrial Co., Ltd.) -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Barium sulfate: Baryte powder available as “Barico #100” from     Hakusui Tech -   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from     Sakai Chemical Co., Ltd. -   Zinc salt of pentachlorothiophenol:     -   Available from Wako Pure Chemical Industries, Ltd.

Formation of Inner and Outer Envelope Layers

Next, in each Example and Comparative Example aside from Comparative Example 6, an inner envelope layer was formed by injection molding the inner envelope layer material of formulation No. 1 or No. 4 shown in Table 2 over the core, following which an outer envelope layer was formed by injection molding the outer envelope layer material of formulation No. 2, No. 3, No. 5, No. 6 or No. 7 shown in Table 2. In Comparative Example 6, the material of formulation No. 2 in Table 2 was injection molded over the core to form a single envelope layer (outer envelope layer).

Formation of Intermediate Layer and Cover (Outermost Layer)

Next, in all of the Examples and Comparative Examples, an intermediate layer was formed by injection molding the intermediate layer material of formulation No. 8, No. 9 or No. 10 shown in Table 2 over the envelope layer-encased sphere obtained above. Next, a cover (outermost layer) was formed by injection molding the cover material of formulation No. 11, No. 12 or No. 13 shown in Table 2 over the resulting intermediate layer-encased sphere in each example. A plurality of given dimples common to all the Examples and Comparative Examples were formed at this time on the surface of the cover.

TABLE 2 Resin composition (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 No. 10 No. 11 No. 12 No. 13 HPF 1000 64 100 64 HPF 2000 100 100 Himilan 1605 36 36 Himilan 1557 50 Himilan 1601 50 Surlyn 9150 50 Surlyn 8150 50 Hytrel 3046 100 50 Hytrel 4001 100 50 11 11 Trimethylolpropane 1.1 Polytail H 4 4 T-8295 100 T-8290 75 37.5 T-8283 25 62.5 Titanium oxide 3.9 3.9 3.9 Polyethylene wax 1.2 1.2 1.2 Isocyanate compound 7.5 7.5 7.5

Trade names of the chief materials in the above table are given below.

-   HPF 1000: Dupont HPF™ 1000 -   HPF 2000: Dupont HPF™ 2000 -   Himilan: Ionomers available from DuPont-Mitsui Polychemicals Co.,     Ltd. -   Surlyn: Ionomers available from E.I. DuPont de Nemours & Co. -   Hytrel: Polyether ester elastomers available from DuPont-Toray Co.,     Ltd. -   Trimethylolpropane: TMP, available from Tokyo Chemical Industry Co.,     Ltd. -   Polytail H: A polyhydroxy hydrocarbon-based polymer available from     Mitsubishi Chemical Corporation -   T-8295, T-8290, T-8283: MDI-PTMG type thermoplastic polyurethanes     available from DIC Covestro Polymer, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd. -   Isocyanate compound: 4,4-Diphenylmethane diisocyanate

Formation of Coating Layer

Next, as a coating composition common to all the Examples and Comparative Examples, Coating Composition I shown in Table 3 below was applied with an air spray gun onto the surface of the cover (outermost layer) on which numerous dimples had been formed, thereby producing golf balls having a 15 μm-thick coating layer formed thereon.

TABLE 3 Coating Base resin Polyester polyol (A) 23 composition I Polyester polyol (B) 15 (pbw) Organic solvent 62 Curing agent Isocyanate (HMDI isocyanurate) 42 Solvent 58 Molar blending ratio (NCO/OH) 0.89 Coating Elastic work recovery (%) 84 properties Shore M hardness 84 Shore C hardness 63 Thickness (μm) 15

Polyester Polyol (A) Synthesis Example

A reactor equipped with a reflux condenser, a dropping funnel, a gas inlet and a thermometer was charged with 140 parts by weight of trimethylolpropane, 95 parts by weight of ethylene glycol, 157 parts by weight of adipic acid and 58 parts by weight of 1,4-cyclohexanedimethanol, following which the temperature was raised to between 200 and 240° C. under stirring and the reaction was effected by 5 hours of heating. This yielded Polyester Polyol (A) having an acid value of 4, a hydroxyl value of 170 and a weight-average molecular weight (Mw) of 28,000.

Next, the Polyester Polyol (A) synthesized above was dissolved in butyl acetate, thereby preparing a varnish having a nonvolatiles content of 70 wt/o %.

The base resin for Coating Composition I in Table 3 was prepared by mixing 23 parts by weight of the above polyester polyol solution together with 15 parts by weight of Polyester Polyol (B) (the saturated aliphatic polyester polyol NIPPOLAN 800 from Tosoh Corporation; weight-average molecular weight (Mw), 1.000; 100% solids) and the organic solvent. This mixture had a nonvolatiles content of 38.0 wt %.

Elastic Work Recovery

The elastic work recovery of the coating material was measured using a coating sheet having a thickness of 50 μm. The ENT-2100 nanohardness tester from Erionix Inc. was used as the measurement apparatus, and the measurement conditions were as follows.

Indenter: Berkovich indenter (material: diamond; angle α: 65.03°)

Load F: 0.2 mN

Loading time: 10 seconds

Holding time: 1 second

Unloading time: 10 seconds

The elastic work recovery was calculated as follows, based on the indentation work W_(elast) (Nm) due to spring-back deformation of the coating and on the mechanical indentation work W_(total) (Nm).

Elastic work recovery=W _(elast) /W _(total)×100(%)

Shore C Hardness and Shore M Hardness

The Shore C hardnesses and Shore M hardnesses in Table 3 above were determined by fabricating the material being tested into 2 mm thick sheets and stacking three such sheets together to form test specimens. Measurements were taken using a Shore C durometer and a Shore M durometer in accordance with ASTM D2240.

Various properties of the resulting golf balls, including the internal hardnesses of the core at various positions, the diameters of the core and each of the layer-encased spheres, the thickness and material hardness of each layer, and the surface hardnesses of the respective layer-encased spheres, were evaluated by the following methods. The results are presented in Tables 4-1 and 4-2.

Diameters of Core. Inner and Outer Envelope Layer-Encased Spheres and Intermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core, inner or outer envelope layer-encased sphere or intermediate layer-encased sphere, the average diameter for ten such spheres was determined.

Ball Diameter

The diameter at 15 random dimple-free areas was measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for ten balls was determined.

Core Deflection

A core was placed on a hard plate and the amount of deflection of the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection refers in each case to the measured value obtained after holding the core isothermally at 23.9° C.

Core Hardness Profile

The indenter of a durometer was set substantially perpendicular to the spherical surface of the core, and the surface hardness of the core on the Shore C hardness scale was measured in accordance with ASTM D2240. Cross-sectional hardnesses at the center of the core and at given positions in each core were measured by perpendicularly pressing the indenter of a durometer against the place to be measured in the flat cross-sectional plane obtained by cutting the core into hemispheres. The measurement results are indicated as Shore C hardness values.

In addition, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the core center and surface, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the surface areas A to F defined as follows

½×2.5×(C _(M−5.0) −C _(M−7.5)),  surface area A:

½×2.5×(C _(M−2.5) −C _(M−5.0)),  surface area B:

½×2.5×(C _(M) −C _(M−2.5)),  surface area C:

½×2.5×(C _(M+2.5) −C _(M)),  surface area D:

½×2.5×(C _(M+5.0) −C _(M+2.5)), and  surface area E:

½×2.5×(C _(M+7.5) −C _(M+5.0))  surface area F:

were calculated, and the values of the following three expressions were determined:

(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C);

(surface area D+surface area E)−(surface area A+surface area B+surface area C);

[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc).

Surface areas A to F in the core hardness profile are explained in FIG. 2, which is a graph that illustrates surface areas A to F using the core hardness profile data from Example 1.

Material Hardnesses (Shore D Hardnesses) of Inner and Outer Envelope Layers, Intermediate Layer and Cover

The resin materials for each of these layers were molded into sheets having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardnesses were measured in accordance with ASTM D2240.

Surface Hardnesses (Shore D Hardnesses) of Inner and Outer Envelope Layer-Encased Spheres, Intermediate Layer-Encased Sphere and Ball

The surface hardnesses were measured by perpendicularly pressing an indenter against the surfaces of the respective spheres. The surface hardnesses of the balls (covers) were values measured at dimple-free areas (lands) on the surface of the ball. The Shore D hardnesses were measured with a type D durometer in accordance with ASTM D2240.

TABLE 4-1 Example Comparative Example 1 2 3 4 1 7 3 4 5 6 7 8 Ball construction 5- 5- 5- 5- 5- 5- 5- 5- 5- 4- 5- 5- piece piece piece piece piece pece piece piece piece piece piece piece Core Diameter (mm) 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 33.3 Weight (g) 24.6 24.6 24.6 24.6 23.9 22.9 24.6 24.5 24.6 24.5 24.6 24.6 Deflection (mm) 4.3 4.7 4.3 4.7 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.7 Core Surface hardness (Cs) 81 77 81 77 81 81 81 81 81 81 76 72 hardness Hardness 7.5 mm toward core 79 75 79 75 79 79 79 79 79 79 73 69 profile surface side from midpoint M (C_(M+7.5)) Hardness 5.0 mm toward core 74 71 74 71 74 74 74 74 74 74 71 67 surface side from midpoint M (C_(M+5)) Hardness 2.5 mm toward core 68 65 68 65 68 68 68 68 68 68 69 66 surface side from midpoint M (C_(M+2.5)) Hardness at midpoint M between 63 62 63 62 63 63 63 63 63 63 67 63 core center and surface (C_(M)) Hardness 2.5 mm toward 60 59 60 59 60 60 60 60 60 60 66 62 core center side from midpoint M (C_(M−2.5)) Hardness 5.0 mm toward core center side 58 57 58 57 58 58 58 58 58 58 64 60 from midpoint M (C_(M−5)) Hardness 7.5 mm toward 55 54 55 54 55 55 55 55 55 55 61 57 core center side from midpoint M (C_(M−7.5)) Center hardness (Cc) 54 52 54 52 54 54 54 54 54 54 60 56 Surface hardness − Center Hardness (Cs − Cc) 27 25 27 25 27 27 27 27 27 27 16 16 Surface area A: ½ × 2.5 × 3.3 3.8 3.3 3.8 3.3 3.3 3.3 3.3 3.3 3.3 3.8 3.8 (C_(M−5) − C_(M−7.5)) Surface area B: ½ × 2.5 × 2.9 2.5 2.9 2.5 2.9 2.9 2.9 2.9 2.9 2.9 2.5 2.5 (C_(M−2.5) − C_(M−5)) Surface area C: ½ × 2.5 × 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 1.3 1.3 (C_(M) − C_(M−2.5)) Surface area D: ½ × 2.5 × 6.3 3.8 6.3 3.8 6.3 6.3 6.3 6.3 6.3 6.3 2.5 3.8 (C_(M+2.5) − C_(M)) Surface area E: ½ × 2.5 × 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 2.5 1.3 (C_(M+5) − C_(M+2.5)) Surface area F: ½ × 2.5 × 6.3 5.0 6.3 5.0 6.3 6.3 6.3 6.3 6.3 6.3 2.5 2.5 (C_(M+7.5) − C_(M+5)) Surface areas A + B + C 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 7.5 7.5 Surface areas D + E 13.8 11.3 13.8 11.3 13.8 13.8 13.8 13.8 13.8 13.8 5.0 5.0 Surface areas D + E + F 20.0 16.3 20.0 16.3 20.0 20.0 20.0 20.0 20.0 20.0 7.5 7.5 (Surface areas D + E + F) − (Surface areas A + 10.0 6.3 10.0 6.3 10.0 10.0 10.0 10.0 10.0 10.0 0.0 0.0 B + C) (Surface areas D + E − (Surface areas A + B + C) 3.8 1.3 3.8 1.3 3.8 3.8 3.8 3.8 3.8 3.8 −2.5 −2.5 [(Surface areas D + E + F) − (Surface areas A + 0.37 0.25 0.37 0.25 0.37 0.37 0.37 0.37 0.37 0.37 0.00 0.00 B + C)]/(Cs − Cc) Surface hardness (Shore D) 47 44 47 44 47 47 47 47 47 47 44 41

TABLE 4-2 Example Comparative Example 1 2 3 4 1 2 Inner Resin material No. 1 No. 1 No. 1 No. 1 No. 1 No. 4 envelope Thickness (mm) 1.4 1.4 1.4 1.4 1.4 1.4 layer Material hardness (sheet hardness: Shore D) 56 56 56 56 56 40 Inner Diameter (mm) 36.1 36.1 36.1 36.1 36.1 36.1 envelope layer-encased Weight (g) 29.6 29.6 29.6 29.6 29.0 28.9 sphere Surface hardness (Shore D) 62 62 62 62 62 47 Outer Resin material No. 2 No. 2 No. 3 No. 3 No. 5 No. 6 envelope Thickness (mm) 1.2 1.2 1.2 1.2 1.2 1.2 layer Material hardness (sheet hardness: Shore D) 47 47 50 50 27 34 Outer Diameter (mm) 3S.5 38.5 38.5 38.5 38.5 38.5 envelope layer-encased Weight (g) 34.6 34.6 34.6 34.6 34.6 34.6 sphere Surface hardness (Shore D) 53 53 56 56 35 41 Intermediate Resin material No. 8 No. 8 No. 8 No. 8 No. 8 No. 8 layer Thickness (mm) 1.3 1.3 1.3 1.3 1.3 1.3 Material hardness (sheet hardness: Shore D) 68 68 68 68 68 68 Intermediate Diameter (mm) 41.1 41.1 41.1 41.1 41.1 41.1 layer-encased Weight (g) 40.8 40.8 40.8 40.8 40.8 40.8 sphere Surface hardness (Shore D) 74 74 74 74 74 74 Cover Resin material No. 11 No. 11 No. 12 No. 12 No. 12 No. 12 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 Material hardness (sheet hardness: Shore D) 47 47 43 43 43 43 Coating layer Material 1 1 1 1 1 1 Shore C hardness (He) 63 63 63 63 63 63 Ball Diameter (Mm) 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 Surface hardness (Shore D) 63 63 62 62 62 62 Hardness Inner envelope layer surface hardness − Core 15 18 15 18 15 0 relationships surface hardness (Shore D) Outer envelope layer surface hardness − −9 −9 −6 −6 −27 −6 Inner envelope layer surface hardness (Shore D) Intermediate layer surface hardness − 21 21 18 18 39 33 Outer envelope layer surface hardness (Shore D) Ball surface hardness − Intermediate −11 −11 −12 −12 −12 −12 layer surface hardness (Shore D) Outer envelope layer surface hardness − 6 9 9 12 −12 −6 Core surface hardness (Shore D) Cm − Hc (Hardness at core midpoint − 0 −1 0 −1 0 0 Coating hardness) Thickness Inner envelope layer − 0.2 0.2 0.2 0.2 0.2 0.2 relationships Outer envelope layer (mm) Intermediate layer − 0.1 0.1 0.1 0.1 0.1 0.1 Outer envelope layer (mm) Inner envelope layer − 0.1 0.1 0.1 0.1 0.1 0.1 Intermediate layer (mm) Intermediate layer − 0.5 0.5 0.5 0.5 0.5 0.5 Cover (mm) Inner envelope layer + 2.6 2.6 2.6 2.6 2.6 2.6 Outer envelope layer (mm) Intermediate layer + 2.1 2.1 2.1 2.1 2.1 2.1 Cover (num) (Inner envelope layer + 0.5 0.5 0.5 0.5 0.5 0.5 Outer envelope layer) − (Intermediate layer + Cover) (mm) Comparative Example 3 4 5 6 7 8 Inner Resin material No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 envelope Thickness (mm) 1.4 1.4 1.4 — 1.4 1.4 layer Material hardness (sheet hardness: Shore D) 56 56 56 — 56 56 Inner Diameter (mm) 36.1 36.1 36.1 — 36.1 36.1 envelope layer-encased Weight (g) 29.6 29.5 29.6 — 29.6 29.6 sphere Surface hardness (Shore D) 62 62 62 — 62 62 Outer Resin material No. 7 No. 3 No. 2 No. 2 No. 7 No. 7 envelope Thickness (mm) 1.2 1.2 1.2 2.6 1.2 1.2 layer Material hardness (sheet hardness: Shore D) 60 50 47 47 60 60 Outer Diameter (mm) 38.5 38.5 38.5 38.5 38.5 38.5 envelope layer-encased Weight (g) 34.6 34.6 34.6 34.6 34.6 34.6 sphere Surface hardness (Shore D) 66 56 53 53 66 66 Intermediate Resin material No. 8 No. 9 No. 10 No. 8 No. 8 No. 8 layer Thickness (mm) 1.3 1.3 1.3 1.3 1.3 1.3 Material hardness (sheet hardness: Shore D) 68 47 56 68 68 68 Intermediate Diameter (mm) 41.1 41.1 41.1 41.1 41.1 41.1 layer-encased Weight (g) 40.8 40.8 40.8 40.8 40.8 40.8 sphere Surface hardness (Shore D) 74 53 62 74 74 74 Cover Resin material No. 12 No. 12 No. 13 No. 12 No. 12 No. 12 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 Material hardness (sheet hardness: Shore D) 43 43 57 43 43 43 Coating layer Material 1 1 1 1 1 1 Shore C hardness (He) 63 63 63 63 63 63 Ball Diameter (Mm) 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 Surface hardness (Shore D) 62 52 63 62 62 62 Hardness Inner envelope layer surface hardness − Core 15 15 15 — 18 21 relationships surface hardness (Shore D) Outer envelope layer surface hardness − 4 −6 −9 — 4 4 Inner envelope layer surface hardness (Shore D) Intermediate layer surface hardness − 8 −3 9 21 8 8 Outer envelope layer surface hardness (Shore D) Ball surface hardness − Intermediate −12 −1 1 −12 −12 −12 layer surface hardness (Shore D) Outer envelope layer surface hardness − 19 9 6 6 22 25 Core surface hardness (Shore D) Cm − Hc (Hardness at core midpoint − 0 0 0 0 4 0 Coating hardness) Thickness Inner envelope layer − Outer envelope layer (mm) 0.2 0.2 0.2 — 0.2 0.2 relationships Intermediate layer − Outer envelope layer (mm) 0.1 0.1 0.1 −1.3 0.1 0.1 Inner envelope layer − Intermediate layer (mm) 0.1 0.1 0.1 — 0.1 0.1 Intermediate layer − Cover (mm) 0.5 0.5 0.5 0.5 0.5 0.5 Inner envelope layer + Outer envelope layer (mm) 2.6 2.6 2.6 2.6 2.6 2.6 Intermediate layer + Cover (mm) 2.1 2.1 2.1 2.1 2.1 2.1 (Inner envelope layer + Outer envelope layer) − 0.5 0.5 0.5 0.5 0.5 0.5 (Intermediate layer + Cover) (mm)

The flight performance (W#1), spin rate on approach shots, feel at impact and scuff resistance of each golf ball were evaluated by the following methods. The results are shown in Table 5.

Flight Performance

A driver (W#1) was mounted on a golf swing robot and the distance traveled by the ball when struck at a head speed of 45 m/s was measured and rated according to the criteria shown below. The club used was the TourStage X-Drive709 D430 Driver/2013 model (loft angle, 9.5°) manufactured by Bridgestone Sports Co., Ltd. In addition, using an apparatus for measuring the initial conditions, the spin rate was measured immediately after the ball was similarly struck.

Rating Criteria

-   -   Good: Total distance was 230.0 or more     -   NG: Total distance was less than 230.0 m

Evaluation of Spin Rate on Approach Shots

A sand wedge was mounted on a golf swing robot and the amount of spin by the ball w en struck at a head speed of 22 m/s was rated according to the criteria shown below. The sand wedge was the TourB XW-1/2018 model (loft angle, 56°) manufactured by Bridgestone Sports Co., Ltd.

Rating Criteria

-   -   Good: Spin rate was 6,200 rpm or more     -   NG: Spin rate was less than 6,200 rpm

Feel

Sensory evaluations were carried out when the balls were hit with a driver (W#1) by skilled amateur golfers having head speeds of 40 to 50 m/s. The feel of the balls was rated according to the following criteria.

Rating Criteria

-   -   Good: Eight or more out often golfers rated the ball as having a         good feel     -   Fair: Five to seven out often golfers rated the ball as having a         good feel     -   NG: Four or fewer out often golfers rated the ball as having a         good feel

Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot and the ball was hit once at a head speed (HS) of 35 m/s, following which the surface state of the ball was visually examined and rated as follows.

Rating Criteria

-   -   Good: The ball can still be used     -   NG: The ball can no longer be used

TABLE 5 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 8 Flight Spin rate 2,853 2,781 2,878 2,800 3,027 3,221 2,832 3111 2,948 2,983 2,933 2,898 (W#1; (rpm) HS, 45 m/s) Total 231.2 230.8 230.7 230.3 227.9 225.2 231.2 227.4 230.0 229.2 229.7 229.4 distance (m) Rating Good Good Good Good NG NG Good NG Good NG NG NG Controllability Spin rate 6,296 6,251 6,341 6,301 6,332 6,298 6,371 6,401 6,083 6,319 6,382 6,359 on approach (rpm) shots Rating Good Good Good Good Good Good Good Good NG Good Good Good Feel Rating Good Good Good Good Good Good NG Good Good Good NG NG Scuff Rating Good Good Good Good Good Good Good Good NG Good Good Good resistance

As demonstrated by the results in Table 5, the golf balls of Comparative Examples 1 to 8 were inferior in the following respects to the golf balls according to the present invention that were obtained in the Examples.

In Comparative Example 1, the surface hardness of the outer envelope layer was lower than the surface hardness of the core. As a result, the ball had an increased spin rate on shots with a driver and thus a poor distance.

In Comparative Example 2, the surface hardness of the inner envelope layer was the same as or lower than the surface hardness of the core, and the surface hardness of the envelope layer was lower than the surface hardness of the core. As a result, the ball had an increased spin rate on shots with a driver and thus a poor distance.

In Comparative Example 3, the surface hardness of the outer envelope layer was higher than the surface hardness of the inner envelope layer. As a result, the feel at impact was too hard.

In Comparative Example 4, the surface hardness of the intermediate layer was lower than the surface hardness of the outer envelope layer. As a result, the spin rate on shots with a driver (W#1) rose and the distance was poor.

In Comparative Example 5, the surface hardness of the ball was higher than the surface hardness of the intermediate layer. As a result, the ball had a lower spin rate on approach shots and a poor controllability in the short game, in addition to which the scuff resistance was poor.

The ball in Comparative Example 6 was a four-piece solid golf ball having a single envelope layer. The spin rate on shots with a driver (W#1) rose, resulting in a poor distance.

In Comparative Example 7, the core hardness profile had a (surface areas D+E+F)−(surface areas A+B+C) value of less than 3. As a result, the spin rate of the ball on shots with a driver (W#1) rose and so the distance was poor. In addition, the surface hardness of the outer envelope layer was higher than the surface hardness of the inner envelope layer and the ball had a feel at impact that was too hard.

In Comparative Example 8, the core hardness profile had a (surface areas D+E+F)−(surface areas A+B+C) value of less than 3. As a result, the spin rate of the ball on shots with a driver (W#1) rose and so the distance was poor. In addition, the surface hardness of the outer envelope layer was higher than the surface hardness of the inner envelope layer and the ball had a feel at impact that was too hard.

Japanese Patent Application No. 2018-229798 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber: the envelope layer, intermediate layer and cover are each formed of a resin material; the envelope layer is formed into two layers—an inner layer and an outer layer; and the core has a center hardness and a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy the following relationship: core center hardness<core surface hardness<surface hardness of inner envelope layer-encased sphere>surface hardness of outer envelope layer-encased sphere<surface hardness of intermediate layer-encased sphere>ball surface hardness.
 2. The golf ball of claim 1, wherein the core has a hardness profile in which, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the center and the surface of the core, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the following surface areas A to F: ½×2.5×(C _(M−5.0) −C _(M−7.5))  surface area A: ½×2.5×(C _(M−2.5) −C _(M−5.0))  surface area B: ½×2.5×(C _(M) −C _(M−2.5))  surface area C: ½×2.5×(C _(M+2.5) −C _(M))  surface area D: ½×2.5×(C _(M+5) −C _(M+2.5))  surface area E: ½×2.5×(C _(M+7.5) −C _(M+5))  surface area F: satisfy the condition (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)≥3.
 3. The golf ball of claim 2, wherein surface areas A to E in the core hardness profile satisfy the condition (surface area D+surface area E)−(surface area A+surface area B+surface area C)≥0.
 4. The golf ball of claim 2, wherein surface areas A to F in the core hardness profile satisfy the condition 0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.
 5. The golf ball of claim 2, wherein surface areas B to E in the core hardness profile satisfy the condition surface area B<surface area C≤surface area D<surface area E.
 6. The golf ball of claim 1, wherein the envelope layer has a thickness which satisfies the condition thickness of inner envelope layer≥thickness of outer envelope layer.
 7. The golf ball of claim 1, wherein the thicknesses of the outer envelope layer and the intermediate layer satisfy the condition thickness of intermediate layer≥thickness of outer envelope layer.
 8. The golf ball of claim 1, wherein the thicknesses of the envelope layer, the intermediate layer and the cover satisfy the condition (combined thickness of inner envelope layer and outer envelope layer)−(combined thickness of intermediate layer and cover)≥0.1 mm.
 9. The golf ball of claim 1, wherein a coating layer is formed on a surface of the cover, which coating layer has a Shore C hardness of from 40 to
 80. 10. The golf ball of claim 9 wherein, letting He be the Shore C hardness of the coating film layer and letting C_(M) be the Shore C hardness at a midpoint M between the center and surface of the core, the difference C_(M)−Hc is at least −10 and up to
 10. 11. A multi-piece solid golf ball comprising a core, an inner envelope layer, an outer envelope layer, an intermediate layer and a cover, wherein the core has a surface hardness, the sphere obtained by encasing the core with the inner envelope layer (inner envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the inner envelope layer-encased sphere with the outer envelope layer (outer envelope layer-encased to sphere) has a surface hardness, the sphere obtained by encasing the outer envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy the following relationship: core surface hardness<surface hardnesses of the inner envelope layer-encased sphere, outer envelope layer-encased sphere and intermediate layer-encased sphere>ball surface hardness, with the proviso that the surface hardness of the outer envelope layer-encased sphere is not more than 60 on the Shore D hardness scale and the surface hardness of the intermediate layer-encased sphere is at least 66 on the Shore D hardness scale; and the core has a hardness profile in which, letting Cc be the Shore C hardness at the core center, Cs be the Shore C hardness at the core surface, C_(M) be the Shore C hardness at a midpoint M between the center and the surface of the core, C_(M+2.5), C_(M+5.0) and C_(M+7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side and C_(M−2.5), C_(M−5.0) and C_(M−7.5) be the respective Shore C hardnesses at positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the following surface areas A to F: ½×2.5×(C _(M−5.0) −C _(M−7.5))  surface area A: ½×2.5×(C _(M−2.5) −C _(M−5.0))  surface area B: ½×2.5×(C _(M) −C _(M−2.5))  surface area C: ½×2.5×(C _(M+2.5) −C _(M))  surface area D: ½×2.5×(C _(M+5) −C _(M+2.5))  surface area E: ½×2.5×(C _(M+7.5) −C _(M+5))  surface area F: satisfy the condition (surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)≥6.
 12. The golf ball of claim 11, wherein surface areas A to F in the core hardness profile satisfy the condition 0.15≤[(surface area D+surface area E+surface area F)−(surface area A+surface area B+surface area C)]/(Cs−Cc)≤0.6.
 13. The golf ball of claim 11, wherein a coating layer is formed on a surface of the cover and, letting He be the Shore C hardness of the coating layer and letting C_(M) be the Shore C hardness at the midpoint M between the center and surface of the core, the difference C_(M)−Hc is at least −10 and up to
 10. 